What happens if I exceed my agreed supply capacity?

Agreed supply capacity (ASC), also called maximum import capacity, is the maximum power your site is allowed to draw, measured in kVA. See supply capacity and kVA charges.

What happens when you go over

An excess capacity charge applies when a half-hourly metered supply draws more than its agreed capacity. It is part of the distribution (DUoS) charges that your supplier passes on. Before April 2018 the excess was billed at the same rate as the normal capacity charge, so many sites never noticed it. An industry change known as DCP161 then raised the excess rate above the standard capacity rate. The rates are set by each distribution network operator and vary by region, so check the figures on your own bill.

Beyond the charge, a regularly overloaded supply can cause tripped breakers or voltage problems, which are signs the connection is too small.

How to check

1. Find the capacity (kVA) stated on your bill or supply details.
2. Look in the distribution section of recent invoices for an excess capacity line.
3. Download half-hourly data to see peak demand against your limit.

Worked example (illustrative)

Your agreed capacity is 100 kVA and a new oven pushes your peak to 115 kVA. You have exceeded it by 15 kVA. Network operators typically charge excess capacity per kVA of excess per day across the billing month in which it happens, so the sum is usually:

excess kVA × excess rate (p per kVA per day) × days in the month

If, for illustration only, the excess rate on your bill were 10p per kVA per day, a 30-day month would cost 15 × £0.10 × 30 = £45.00, on top of the normal capacity charge on the full 100 kVA. Take the real rate from the distribution section of your own bill; this example just shows the shape of the calculation.

Your options

• Reduce peaks by staggering start-up of equipment, or by adding monitoring (see energy monitoring).
• Increase your agreed capacity by asking your supplier or network operator. Any cost depends on whether the local network has spare capacity, so get a written quote first.
• Reduce capacity if you pay for far more than you use, which may lower fixed charges; see non-commodity charges.

Cutting capacity too far invites excess charges. Review a full year of data first.

Does this affect non-half-hourly sites?

The excess capacity charge applies to half-hourly metered supplies, which usually means larger sites. A small shop or office on a standard meter does not normally have an agreed capacity charge on its bill. If you have recently been moved to half-hourly metering, check whether an agreed capacity has been set for your site and whether it matches what you actually need; see half-hourly meters explained.

If you think the charge is wrong

Ask your supplier for the data behind it, then follow disputing a bill.

How to lower the risk

• Check peak demand after installing new equipment, such as ovens, heat pumps or EV chargers.
• Use load management on chargers to share capacity.
• Review your capacity annually using half-hourly data.

What to ask your supplier

1. What agreed capacity is on my account?
2. How many half-hours did I exceed it last year?
3. What did I pay in excess capacity charges?

Is the charge avoidable after the fact?

Charges follow what actually happened, so past excesses generally stand unless there's an error in the data or the agreed capacity recorded. If you believe the recorded capacity is wrong, ask your supplier to check it with the network operator and see refunds and credits if you were overcharged.